polyomavirusany of a group subgroup of minute oncogenic DNA viruses of the family Polyomaviridae. Together with papillomaviruses, polyomaviruses were once classed as members of the Papovaviridae, no longer a valid virus family.

The virus was first isolated in 1953 , when the murine polyomavirus was discovered to have caused tumours in laboratory mice. Since then the virus has been found in a wide variety of vertebrates, from green monkeys and baboons to cage birds (notably those of the parrot family, Psittacidae) and cows. Two rare human polyomaviruses also were isolated , in 1971 and are known as BK virus and JC virus. Infection with BK virus may cause mild respiratory disease, whereas infection with JC virus can affect the respiratory system, the kidneys, or the brain. JC virus is responsible for causing progressive multifocal leukoencephalopathy (PMLE) in immunocompromised people.

Another example of a polyomavirus is Simian vacuolating virus 40 (SV40), which can infect humans, rodents, and monkeys. In some cases, SV40 infection in humans may lead to the growth of malignant tumours.

The polyomavirus is highly antigenic; i.e., all animals bearing its tumours also have virus-neutralizing antibodies in their blood. The virus is also capable of clumping red blood cells and affecting deoxyribonucleic acid (DNA) synthesis.